1. Field of the Invention
This invention generally relates to solid state photodetectors and, more particularly, to a cell structure of photodiodes made using a vertical pillar aligned with the path of light.
2. Description of the Related Art
The most widely used semiconductor devices for light detection are PN or PIN photodiodes. To enable the fastest electrical output signal time response (highest bandwidth) using a photodiode of a particular area, it is necessary to set an optimal thickness of active layer to balance transit time and RC time contributions to the bandwidth (BW). The optimal thickness of active layer with respect to BW is insufficient for the detection of light, as the absorption length in a thick active layer results in low quantum efficiency (QE) of photodetection at longer wavelengths. It is for this reason that silicon PN and PIN photodiodes are typically too inefficient for high-speed optical communications at wavelengths of 850 nanometers (nm) or greater.
FIG. 1 is a diagram depicting a metal-semiconductor-metal (MSM) photodetector (prior art). The MSM photodiode uses the rectifying properties of Schottky contacts between metal and semiconductor materials in a manner similar to PN junctions. The trade-off between transit time and RC time contributions is partially resolved in MSM photodetectors due to the low capacitance of the interdigital electrodes, as compared to the capacitance of PIN photodiodes having the same active area and active layer thickness. Nevertheless, the thickness of the active layer in an MSM device cannot be made too great due to inefficient and slow collection of photoelectrons and holes generated at depths larger than the spacing between fingers. Capacitance can be decreased, and BW increased if the metal electrodes are moved closer together. However, this results in greater shading of the absorption (active) layer. Thus, MSM photodetectors typically use a very thin active layer if high BWs are required, resulting in low QE for wavelengths with an absorption depth longer than the active layer thickness. Materials such Si, Ge, SiC, GaAs, InGaAs, and InP are typically used as the active absorptive layer.
FIG. 2 is a partial cross-sectional view of lateral trench detector (prior art). Improvement in the QE of a MSM photodetector using a relatively thick absorption/active layer can be realized using the so-called lateral trench detector (LTD). The main drawback of the LTD is the high capacitance of parallel trench electrodes, resulting in lower BW. The capacitance is proportional to the length and height of the fingers.
It would be advantageous if a photodetector could be fabricated with a reduced capacitance, to increase the signal bandwidth at long wavelengths of light, without seriously degrading the QE.